Optical transmission analysis may be performed to determine the presence and/or concentration of a substance in a sample based on the interaction of electromagnetic radiation with the sample. In practice, optical transmission analysis may be performed by consideration of a ratio of the intensity of the radiation detected after passing through the sample to the initial intensity of the radiation incident on the sample i.e. the transmittance through the sample. Typically, the analysis considers the absorbance of the sample, which may be determined from the transmittance, and is proportional to the concentration of the absorbing substance, and the optical path length traveled by the radiation through the sample.
For a given wavelength of incident radiation, the concentration of a substance may be determined based on the initial power (Io) of the radiation entering a transmission cell containing the substance, and the power of the radiation detected after passing through the cell (IT), where the radiation has traveled along a path length L through the sample, according to the following relationship;IT=Io·e−α·c·L where c is the concentration of the absorbing substance, α is an absorption coefficient and L is the path length. This is commonly referred to as the Beer-Lambert law.
Taking the log of the detected power gives the convenient linear relationship:log(IT)=log(IO)−α·c·L 
As may be seen from the relationship above, the optical path length through the sample is a key parameter, controlling the degree to which radiation is absorbed by the sample. The extent to which radiation is absorbed also depends upon the nature of the test sample, and the wavelength of the radiation. For example, liquid samples tend to absorb radiation relatively strongly in the infrared region, which is most commonly used in optical analysis of samples, and therefore a relatively small path length is required, for example, commonly used path lengths being in the order of 50-250 micrometers. While the present invention is not limited to the use of infrared radiation, as this is the most commonly used wavelength, which is associated with a convenient path length, it is important to be able to provide an apparatus that is particularly effective for use with such a wavelength range.
Various types of apparatus may be used to perform measurements on samples for use in carrying out optical analysis of the sample based on transmission. One technique, which is applicable to measuring liquid samples, is based on Attenuated Total Reflectance (ATR). In ATR based methods, a liquid sample to be tested is placed in contact with a transparent guide material of higher refractive index. A beam of electromagnetic radiation, typically infrared radiation, is caused to pass through the transparent material, and reflect from the interface of the transparent material which contacts the sample. This process results in an evanescent wave extending a small distance into the test liquid, which is subject to absorption. The effective path length is determined by the angle of reflection, the wavelength of the radiation, and the refractive indices of the test liquid and guide material. In general, such techniques are only appropriate for a limited number of substances, which have strong absorption properties, as in practice the path length that may be defined may be limited to only a few micrometers.
Typically, in order to perform transmission analysis on a liquid sample, the sample is held in a transmission cell bounded by windows transparent to electromagnetic radiation of the wavelength to be used in testing, enabling radiation to pass through one of the windows from a source, and be detected on the other side of the cell after passing through the opposite window. The transmission cell is constructed to provide a well-defined path length through the sample. Transmission cells provide a convenient way to provide a suitable, repeatable and well defined path length through a liquid sample. Transmission cells may be used for online testing, in which a sample liquid is caused to flow through the cell, or for testing of extracted samples.
Difficulties may arise in cleaning the cell after a test, and before testing of a new sample. This may be the case in particular where the path lengths are of the small dimensions often required for liquid analysis e.g. of the order of no more than a few hundred micrometres. Problems with cleaning are exacerbated in the case of viscous or dirty fluids, such as lubricating oils, and the cleaning process may take a considerable amount of time, often greater than the time required to perform a measurement. It is important to ensure that the cell is thoroughly cleaned, as even a small amount of contamination from a previous test may significantly affect a subsequent measurement, especially where small path lengths are involved. There may also be difficulties in introducing the fluid to be tested to the cell due to the small dimensions.